Monosaccharides cyclic hemiacetals/hemiketals

This is a modified form of the 1980 recommendations [4]. Priority is now given to naming cyclic forms, since in most cases branched-chain monosaccharides will form cyclic hemiacetals or hemiketals. 

If the branched monosaccharide forms a cyclic hemiacetal or hemiketal, the chain which includes the ring atoms rather than any alternative open chain must be the basis of the name. Otherwise the parent is chosen according to the principles given in 2-Carb-2.1. 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

When a monosaccharide forms a cyclic hemiacetal (or hemiketal), the carbonyl carbon becomes a stereocenter. Thus, cyclization leads to formation of two possible stereoisomers. These isomers are called anomers, and the former carbonyl carbon is called the anomeric carbon. The isomer with the anomeric OH (shown in blue) pointed down is the alpha anomer. The isomer with the anomeric OH (shown in blue) pointed up is the beta anomer. (Figure 12.14)... 

Hemiacetals and hemiketals react with alcohols to form acetals and ketals, respectively. When the cyclic hemiacetal or hemiketal form of a monosaccharide reacts with an alcohol, the new linkage is called a glycosidic linkage, and the compound is called a glycoside. 

Hemiacetals and hemiketals are readily formed in carbohydrates. Monosaccharides contain several hydroxyl groups and one carbonyl group. The linear form of a monosaccharide quickly undergoes an intramolecular reaction in solution to give a cyclic hemiacetal or hemiketal. 

The actual structure of monosaccharides such as o-glucose is not what has been shown up to this point. We previously learned that aldehydes and ketones react reversibly with alcohols to form hemiacetals and hemiketals, respectively. (Refer back to Sec. 14.7 to review this reaction.) This reaction takes place intramolecularly with monosacchardes since both OH and C = O are present in the same molecule. The structures of aldoses and ketoses are cyclic hemiacetals and hemiketals, respectively. 

Ans. The cyclic hemiacetal and hemiketal structures have no carbonyl function but are in equilibrium with the open-chain carbonyl structure. As the small amount of the open-chain carbonyl structure reacts, some of the a- and /3-anomers undergo ring-opening to replenish the equilibrium concentration of the open-chain structure. The newly formed open-chain structure undergoes reaction. More a- and -anomers open up. There is a continuous shifting of a- and /3-anomers to the open-chain carbonyl structure until all of the monosaccharide initially present has reacted. 

Although carbohydrate cyclic hemiacetals and hemiketals are in equilibrium with open-chain forms of the monosaccharides, the glycosides (acetals and ketals) are much more stable and do not exhibit open-chain forms. Therefore, glycosides of monosaccharides are not reducing sugars. 

Monosaccharides are sweet-tasting solids that are very soluble in water. Noncarbohydrate low-calorie sweeteners such as aspartame have been developed as sugar substitutes. Pentoses and hexoses form cyclic hemiacetals or hemiketals whose structures can be represented by Haworth structures. Two isomers referred to as anomers (the a and p forms) are produced in the cyclization reaction. All monosaccharides are oxidized by Benedict s reagent and are called reducing sugars. Monosaccharides can react with alcohols to produce acetals or ketals that are called glycosides. 

Fischer projections are especially useful for the acyclic forms of monosaccharides (e.g., D-glucose, 60) and have been adapted to display their cyclic hemiacetal or hemiketal forms as shown with 64d (Figure 2.29). Again, the right angles in 64d present the same issues as those in 66a. To avoid these problem. , current lUPAC recommendations recommend the use of figures such as 64e and 64f. ... 

Even though monosaccharides exist largely as cyclic hemiacetals or hemiketals, rapid equilibrium, with trace quantities of carbonyl-containing compounds, means that discussion of the reactions of most sugars must consider both the acyclic and the cyclic structures. Common monosaccharides can be reduced to alkanepolyols (D-manitol, sorbitol) or oxidized to aldaric, aldonic, or uronic acids. 

Although five- and six-carbon monosaccharides exist predominately as hemiacetals and hemiketals, they undergo the characteristic reduction and oxidation reactions of simple aldehydes and ketones. The reduction or oxidation reaction occurs by way of the carbonyl group in the small amount of the open-chain form of the monosaccharide in equilibrium with its cyclic hemiacetal or hemiketal. As the reduction or oxidation occurs, the equilibrium shifts to produce more of the carbonyl form until eventually all the monosaccharide reacts. 

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

Monosaccharides, polyhydroxy aldehydes or ketones, are either aldoses or ketoses. Sugars that contain four or more carbons primarily have cyclic forms. Cyclic aldoses or ketoses are hemiacetals and hemiketals, respectively. 

In Section 4.3, we learned that hemiacetals and hemiketals can react with alcohols in acid solutions to yield acetals and ketals, respectively. Thus, cyclic monosaccharides (hemiacetals and hemiketals) readily react with alcohols in the presence of acid to form acetals and ketals. The general name for these carbohydrate products is glycosides. The reaction of a-D-glucose with methanol is shown in Reaction 7.6. 

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